Explain how to specify a subprotocol

[egekorkan]

#249 moved subprotocols back from the appendix however the current bindings do not explain that clearly. The CoAP binding does it at https://w3c.github.io/wot-binding-templates/bindings/protocols/coap/index.html#observing-resources but not under a special section.
We should update the template as well.

The overall mechanism is also a bit fuzzy, further discussion is needed. We should specify what a subprotocol can specify and what it cannot.

Another alternative would be to remove the subprotocol mechanism and make everything more descriptive (more vocabulary items).

Something we really should focus on (with subprotocol or not) is how to use WebSockets in TDs. This needs a separate discussion which can involve more redesign. Also how do we handle MQTT over WS? Are there other protocols that do this? How to handle RPC in general?

Coming from call of 22.02.